1. Field of the Invention
This invention relates to an apparatus for reading out a radiation image which has been stored on a stimulable phosphor sheet. This invention particularly relates to a radiation image read-out apparatus with which light emitted by a stimulable phosphor sheet in proportion to the amount of energy stored thereon during its exposure to radiation can be detected accurately.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Pat. Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light, which is emitted by the stimulable phosphor sheet when it is stimulated, is photoelectrically detected and converted into an electric image signal. The image signal is then used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
FIG. 4 is a perspective view showing a conventional radiation image read-out apparatus. It has been proposed to use radiation image read-out apparatuses like that shown in FIG. 4 in the radiation image recording and reproducing systems described above. With such a radiation image read-out apparatus, a stimulable phosphor sheet, on which a radiation image has been stored, can be scanned with stimulating rays, such as a laser beam, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and the emitted light can be photoelectrically detected.
In the radiation image read-out apparatus of FIG. 4, stimulating rays 2 are produced by a stimulating ray source 1, and the beam diameter of the stimulating rays 2 is precisely adjusted by a beam expander 3. Thereafter, stimulating rays 2 are deflected by a light deflector 4, which may be constituted of a galvanometer mirror or the like. The deflected stimulating rays 2 are reflected by a plane reflection mirror 5, after which they impinge upon a stimulable phosphor sheet 10 and scan it in the main scanning directions indicated by the double headed arrow A. An f.theta. lens 6 is located in the optical path of the deflected stimulating rays 2 between the light deflector 4 and the plane reflection mirror 5. The f.theta. lens 6 keeps the beam diameter of the stimulating rays 2 uniform, and stimulating rays 2 having a uniform beam diameter scan the stimulable phosphor sheet 10 at a constant speed in the main scanning directions. In the illustrated radiation image read-out apparatus, the stimulating ray source 1, the beam expander 3, the light deflector 4, the plane reflection mirror 5, and the f.theta. lens 6 constitute a main scanning means. While the stimulating rays 2 impinge upon the stimulable phosphor sheet 10, the stimulable phosphor sheet 10 is moved by a sub-scanning means, which may be constituted of an endless belt device 20, in the sub-scanning direction indicated by the arrow B, which direction is approximately normal to the main scanning directions. Consequently, the whole surface of the stimulable phosphor sheet 10 is exposed to the stimulating rays 2. When it is being exposed to the stimulating rays 2, the stimulable phosphor sheet 10 emits light in proportion to the amount of energy stored thereon during its exposure to radiation. The emitted light enters a light guide member 8. The light guide member 8 has a linear light input face 8a which is positioned along a main scanning line 2c on the stimulable phosphor sheet 10. A ring-shaped light output face 8b of the light guide member 8 is connected to a light receiving face of a photodetector 9, which may be constituted of a photomultiplier. The light guide member 8 is made from a transparent thermoplastic resin sheet, such as an acrylic resin sheet, so that light which has entered the light guide member 8 at its light input face 8a is guided through repeated total reflection inside of the light guide member 8 to the light output face 8b. The light, which has been emitted by the stimulable phosphor sheet 10 and which has entered the light guide member 8, is guided inside of the light guide member 8, emanates from the light output face 8b, and is detected by the photodetector 9. Shapes and materials which are suitable for the light guide member 8 are disclosed in, for example, U.S. Pat. No. 4,346,295.
A filter (not shown) is positioned so that it is in close contact with the light receiving face of the photodetector 9. The filter transmits only light whose wavelengths fall within the wavelength distribution range of the light emitted by the stimulable phosphor sheet 10, and filters out light whose wavelengths fall within the wavelength distribution range of the stimulating rays 2. Therefore, only light which is emitted by the stimulable phosphor sheet 10 will be detected by the photodetector 9. The photodetector 9 converts the light emitted by the stimulable phosphor sheet 10 into an electric signal and feeds it into an image processing circuit 11 which processes the electric signal. The processed electric signal is fed into an image reproducing apparatus, which may be constituted of a CRT, a light beam scanning recording apparatus, or the like, and is used to reproduce a visible image. Alternatively, the processed electric signal may be stored on a magnetic tape or the like.
Also, in the radiation image read-out apparatus described above, in order to improve the efficiency with which the light emitted by the stimulable phosphor sheet 10, when it is scanned with the stimulating rays 2, is guided, a reflection mirror 14 is often located facing the light input face 8a of the light guide member 8 with the main scanning line 2c intervening therebetween. The reflection mirror 14 reflects the light which is emitted from the position on the stimulable phosphor sheet 10 which is being scanned. The light advances from the side opposite to the light guide member 8 towards the light input face 8a of the light guide member 8.
FIG. 5 is a schematic side view showing a major part of the radiation image read-out apparatus shown in FIG. 4. As illustrated in FIG. 5, in the conventional radiation image read-out apparatus described above, a part 2a of the stimulating rays 2 impinging upon a position on the stimulable phosphor sheet 10, which position is being scanned at any given instant, is reflected by the surface of the stimulable phosphor sheet 10. These reflected stimulating rays 2a are then reflected by the light input face 8a of the light guide member 8 and impinge upon a position on the stimulable phosphor sheet 10 which is not being scanned at the given instant, which causes said position on the stimulable phosphor sheet which is not being scanned to emit light. (This phenomenon is referred to as the flare phenomenon.) When the flare phenomenon occurs, light emitted from the position on the stimulable phosphor sheet 10, which position is not being scanned at the given instant, enters the light guide member 8 and is detected by the photodetector 9 together with the light which is emitted from the position on the stimulable phosphor sheet 10, which position is being scanned at the given instant. Therefore, the radiation image stored on the stimulable phosphor sheet 10 cannot be read out accurately, and the contrast in a visible radiation image which is reproduced from the image signal thus detected will be low.
Also, in cases where the reflection mirror 14 is located as illustrated in FIG. 5, parts 2a2a of the stimulating rays 2, which have been reflected from the position on the stimulable phosphor sheet 10, which position is being scanned at the given instant, toward a reflection surface 14a of the reflection mirror 14, are reflected by the reflection surface 14a towards the light input face 8a of the light guide member 8. The reflected stimulating rays 2a2a are even further reflected by the light input face 8aand impinge upon positions on the stimulable phosphor sheet 10, which positions are not being scanned at the given instant. Therefore, when the reflection mirror 14 is provided, an even larger adverse effect occurs from the flare phenomenon, even though the efficiency, with which the light emitted by the stimulable phosphor sheet 10 is guided, is improved.
In order to reduce the adverse effects of the flare phenomenon, the applicant has proposed various radiation image read-out apparatuses.
For example, in U.S. Pat. No. 4,818,880, a radiation image read-out apparatus is disclosed wherein an antireflection film is overlaid on a light input face of the light guide member in order to prevent stimulating rays from being reflected by the light input face. Stimulating rays reflected from a stimulable phosphor sheet are allowed to enter the light guide member and are filtered out by a filter, which is positioned between a light output face of the light guide member and a light receiving face of a photodetector. Also, U.S. Pat. No. 4,680,473 discloses a radiation image read-out apparatus wherein an antireflection film, which will prevent stimulating rays from being reflected by a reflection mirror, is overlaid on a reflection surface of the reflection mirror.
In the disclosed radiation image read-out apparatuses wherein an antireflection film is overlaid on a light input face of a light guide member or on a reflection surface of a reflection mirror, the antireflection characteristics of the antireflection film vary in accordance with the angle of incidence of the stimulating rays, which have been reflected from a stimulable phosphor sheet, upon the antireflection film. It has heretofore been considered to be important that stimulating rays, which have been reflected from a stimulable phosphor sheet and which impinge at a comparatively small angle of incidence upon the light input face of the light guide member or upon the reflection surface of the reflection mirror, be prevented as much as possible from being reflected by the light input face or the reflection surface. Therefore, the antireflection film has heretofore been designed so that its reflectivity is lowest for the reflected stimulating rays, which impinge at an angle of incidence of 0.degree. upon the antireflection film. However, it has recently been revealed that stimulating rays, which are reflected from a position on a stimulable phosphor sheet and impinge at a large angle of incidence upon the light input face of the light guide member or upon the reflection surface of the reflection mirror, have an even greater adverse effect upon the image quality of an image, which is reproduced from an image signal detected from the stimulable phosphor sheet, than those stimulating rays, which are reflected from the stimulable phosphor sheet and impinge at a comparatively small angle of incidence upon the light input face of the light guide member or upon the reflection surface of the reflection mirror. Specifically, stimulating rays, which have been reflected from a position on a stimulable phosphor sheet, which is being scanned at any given instant, and impinge at a large angle of incidence upon the light input face of the light guide member, are reflected by the light input face of the light guide member, and impinge upon and stimulate positions on the stimulable phosphor sheet, which are spaced far apart from the position on the stimulable phosphor sheet which is being scanned at the given instant. Also, stimulating rays, which have been reflected from the position on the stimulable phosphor sheet, which is being scanned at any given instant, and impinge at a large angle of incidence upon the reflection surface of the reflection mirror, are reflected by the reflection surface of the reflection mirror to the light input face of the light guide member, are then reflected by the light input face of the light guide member, and impinge upon and stimulate positions on the stimulable phosphor sheet, which are spaced far apart from the position on the stimulable phosphor sheet which is being scanned at the given instant. For example, if the stimulating rays thus reflected from the light input face of the light guide member stimulate positions on the stimulable phosphor sheet, which are spaced far apart from the position on the stimulable phosphor sheet which position is being scanned at any given instant, the change in image density will not be sharp at regions of the image where it should be sharp, or black lines will appear along directions in which the flare phenomenon has occurred. Therefore, the conventional antireflection film cannot substantially eliminate the problem of stimulating rays being reflected from a stimulable phosphor sheet and impinging at a large angle of incidence upon the light input face of the light guide member or upon the reflection surface of the reflection mirror.